The present invention relates to an inner drum exposure apparatus that records an image by scanning a recording material held on the arcuate inner peripheral surface of a support with a light beam that was modulated in accordance with image information and deflected by a total reflection mirror plane of a rotatably driven scanning means and, more specifically, to an inner drum exposure apparatus that is capable of consistent halftone recording.
An inner drum exposure apparatus is generally used in which a light beam such as a laser beam is guided to the photosensitive surface of a recording material held on the inner peripheral surface of a cylindrical or drum-shaped support to scan the recording material through exposure (see, for example, JP 10-133132 A). The recording material on which an image has been recorded through exposure is processed as needed in an automatic developing machine, whereby a latent image formed on the recording material is converted to a visible image.
When the halftone is represented through exposure of a recording material, the inner drum exposure apparatus commonly represents the halftone in a dot shape through AM screening (technique to form a halftone dot image into a gradation image). In AM screening, the minimum unit of a halftone dot image is composed of a large number of dots, for example, 14 (horizontal dots)×14 (vertical dots), i.e., 196 dots in total. Such a halftone dot image is arranged on a two-dimensional plane to record a gradation image. However, when AM screening is used to represent the halftone, moire fringes or tone jump (discontinuous change in gradation) may often occur.
FM screening is also available as a technique to form a halftone dot image into a gradation image. FM screening represents gradations in a recorded image depending on how densely dots of indefinite shape having no regularity are packed. The gradations are represented by arranging an image composed of a relatively small number of dots, e.g., 2×2 dots, i,e., 4 dots in total, on a two-dimensional plane in a dispersed manner. FM screening is advantageous in that moiré fringes can be suppressed based on its principle.
For this reason, also in an inner drum exposure apparatus capable of achieving high productivity, it has been desired to represent the halftone with small dots through FM screening.
However, in a usual inner drum exposure apparatus configured so that a light beam emitted from a laser light source of a single lateral mode is imaged on the scanning surface of a recording material to form an image, a beam spot is formed so as to exhibit a Gaussian distribution. When performing pixel recording through exposure with a beam spot exhibiting such a Gaussian distribution, the device is usually designed such that the half-width of the beam spot can be equal to or larger than the pixel size to prevent gaps from being formed between scanning lines.
In the case of an image recorded by exposing through FM screening with a light beam having a spot whose edge portion is relatively gentle, the size of recording pixels is changed due to fluctuations in optical power, the degree to which development is effected by an automatic developing machine or other factor, which may abruptly change the ratio in a halftone dot image (halftone dot area ratio), leading to large gradation changes. The above design is done to prevent such a situation.
However, the above measure does not solve the problem that it is difficult to represent the halftone with small dots by using FM screening in an inner drum exposure apparatus, because the beam spot is formed so as to exhibit a Gaussian distribution.
In this regard, U.S. Pat. No. 6,121,996 discloses a technique for directly imaging a light source image from a light source retaining a linear light-emitting surface. However, application of this technique to an inner drum exposure apparatus requires mounting a light source above a deflector and rotating it to allow a linear spot orthogonal to the scanning direction to be formed at an arbitrary position on the inner drum, and as a result this involves an increase in the size of the deflector and a reduction in the scanning speed.